1. Field of the Invention
The present invention relates to a wavelength calibration module and technique, and particularly to an absolute wavelength calibration module which optically absorbs at known wavelengths and detects such optical absorption when a narrowed emission band of an excimer or molecular laser is incident on the module.
2. Discussion of the Related Art
Excimer lasers emitting pulsed UV-radiation are becoming increasingly important instruments in specialized material processing. The KrF-excimer laser emitting around 248 nm and the ArF-excimer laser emitting around 193 nm are rapidly becoming the light sources of choice for photolithographic processing of integrated circuit devices (IC""s). The F2-laser is also being developed for such usage and emits light around 157 nm.
It is important for their respective applications to the field of submicron silicon processing that each of the above excimer laser systems become capable of emitting a narrow spectral band around a very precisely determined and finely adjustable absolute wavelength. Techniques for reducing bandwidths by special resonator designs to less than 100 pm, and in some cases to less than 1 pm, are well known. Techniques are also available for tuning and controlling central wavelengths of emission. However, most of these techniques do not accurately determine absolute wavelengths and only serve to relatively tune and control wavelengths. Moreover, even relative wavelength changes cannot be as precisely determined as is desired, using these techniques.
It is possible to roughly determine an absolute wavelength or a change in wavelength from a reference wavelength as a spectral band is tuned, when particular incremental settings of a spectrograph are calibrated to correspond to absolute wavelengths in conventional units, e.g., in nanometers. However, conventional techniques do not provide very precise absolute wavelength and incremental wavelength change information at any time. This is because a conventional spectrograph often must undergo a laborious conventional calibration technique. Moreover, optical drift and other optical, thermal and electronic phenomena produce uncertainty and imprecision at all times following the calibration procedure, including during operation of the system. Further, wavelength calibration is usually done externally to the operating beam path of the system using high resolution spectrographs in combination with spectral reference tools for wavelength calibration, e.g., spectral lamps emitting particular narrow lines. Therefore, very precise and temporally advantageous absolute wavelength determination and fine tuning methods are needed.
Specifically, it is desired to have absolute wavelength calibration techniques for UV-emitting excimer and molecular lasers having accuracies within a range of xc2x10.25 pm, while having tuning versatility comprising wavelength ranges from xc2x15 pm to greater than xc2x1100 pm depending on properties of available illumination tools for IC production. There are available techniques for accurately determining the absolute wavelength of a narrow band KrF-excimer laser emission using narrow spectral absorption lines of certain elements to calibrate a high resolution spectrometer. Among these available techniques, atomic transition(s) of iron (Fe) at 248.327 and/or 248.4185 nm are used to detect absorption signals either by reduced optical transmission or using the opto-galvanic effect. See U.S. Pat. No. 4,823,354 to Znotins et al.; U.S. Pat. No. 5,450,207 to Fomenkov; F. Babin et al., Opt. Lett., v. 12, p. 486 (1987); See also R. B. Green et al., Appl. Phys. Lett., v. 29, p. 727 (1976) (describing galvanic detection of optical absorptions in a gas discharge for various gases including lithium (Li), sodium (Na), uranium (U) and barium (Ba)).
Babin et al. discloses using the opto-galvanic effect to determine the KrF-laser absolute emission wavelength. A galvatron having an anode and a cathode is set in the optical path of the laser beam. An Fe vapor fills the galvatron. A voltage is monitored between the cathode and anode. The emission wavelength of the laser is narrowed and tuned a range around 248 nm. When the wavelength of the beam impinging the Fe-vapor filled gas volume between the cathode and the anode corresponds to an atomic transition of Fe, a resonance between the levels causes a marked change in voltage between the anode and cathode. Since the absorption lines of Fe are well known and consistent, the absolute wavelength of the narrowed laser emission band is determinable.
Znotins et al. and Fomenkov each disclose using a photodetector to detect the intensity of light emitted from a KrF-laser. Znotins et al. discloses to use a galvatron having benzene vapor inside. Fomenkov discloses to use a galvatron having an Fe cathode inside. The cathode of Fomenkov gives off Fe vapor which fills the galvatron when a current is generated between the cathode and an associated anode. Light emitted from the KrF-laser traverses the gaseous benzene or iron medium of the galvatron before impinging the photodetector. When the wavelength corresponds to an atomic transition of the gas medium of the galvatron, the gas absorbs the light, and the intensity of light detected is reduced. Thus, the absolute wavelength of emission of the KrF-laser is determinable in also determinable in this alternative way.
The opto-galvanic effect described by Babin et al. and acknowledged by Fomenkov permits a very precise and reliable determination of an absolute emission wavelength of a KrF-excimer laser system. See U.S. Pat. No. 4,905,243 to Lokai et al. A known technique uses sealed hollow cathode lamps containing Fe-vapor in a Ne-buffer gas environment. See Hammamatsu Datasheet: Opto-Galvanic Sensor, Galvatron L 2783 Series, November 89, Japan. Thus, the Fe-lamp has become an important and reliable measuring tool for absolute wavelength calibration for KrF-lithography laser systems in the 248 nm spectral region.
It is desired to have absolute wavelength calibration capability for excimer and molecular fluorine laser systems, and particularly for ArF-lithography laser systems emitting in the 193 nm spectral region, as well as for F2 lithography laser systems emitting in the 157 nm spectral region, having a precision of xc2x10.25 pm. It is further desired to have tuning versatility for ArF-laser systems in a range from around 192.5 to 194 nm, and more particularly from around 193.0 to 193.6 nm, while maintaining precise absolute wavelength calibration. It is also desired to have a calibration technique for an excimer or molecular fluorine laser system that is fast and reliable and is easily performed without complicated additional optical alignment of the beam path, such as one utilizing a spectral reference tool which is fixed and integrated into the optical beam path of the system.
The present invention encompasses these desired features and others by providing a system having a wavelength calibration module for measuring an absolute wavelength of a narrowed spectral emission band of an excimer or molecular laser such as an ArF-excimer laser system or a molecular fluorine (F2) laser. The narrowed spectral emission interacts with the module at one or more specific absolute wavelengths and effects of the interactions are detected, when a component of the narrowed emission is directed through the module at the one or more absolute wavelengths. The absolute wavelength of the narrowed spectral emission is precisely determined when the effects of the interaction are detected. For the ArF-excimer laser system, the module contains an element having a level-transition line around 193 nm and is preferably a gaseous vapor selected from the group of materials consisting of arsenic, gaseous hydrocarbons, halogenized hydrocarbons, carbon contaminated inert gases, carbon, oxygen, germanium, and platinum vapor, as well as other metal vapors including iron. For the F2-laser system, the module contains an element having a level-transition line around 157 nm and is preferably a gaseous vapor selected from the group of materials consisting of gaseous hydrocarbons, halogenized hydrocarbons, oxygen, selenium, bromine and silicon.
The present invention further encompasses the above desired features and others by preferably providing a galvatron having a cathode and an anode as the wavelength calibration module. Preferably, the narrowed emission is directed toward the galvatron and voltage changes are detected between the cathode and anode of the galvatron due to opto-galvanic resonance effects when the narrowed emission is tuned within the broadband ArF-excimer laser emission spectrum or within the F2-laser emission spectrum. Alternatively, a photodetector is aligned to monitor the intensity of light traversing the gaseous medium of the galvatron as the wavelength of the light in tuned; the detected intensity showing marked reduction around absorption-transition lines of the gaseous medium. The absolute wavelength of the narrowed emission is then precisely determined anywhere within the broadband spectrum.
The system preferably further includes an ArF-excimer or molecular fluorine laser chamber and optical resonator, a spectral narrowing and tuning unit, and a detection and control unit for relative wavelength detection, control and tuning. The narrow spectral emission band of the ArF- or F2-laser is directed through an output coupling mirror of the optical resonator and toward the wavelength calibration module.
In another embodiment, a wavelength calibration system is provided for measuring an absolute wavelength of a narrowed spectral emission band of an excimer or molecular laser. In this embodiment, the excimer or molecular laser is preferably selected from the group of lasers consisting of a KrF laser, an ArF laser and a F2 laser. The alternative system includes a purge chamber filled with a photo-absorbing element enclosing optical elements of the system. The photo-absorbing element is preferably oxygen gas or iron metal vapors when the system includes an ArF-excimer laser chamber. Reduction in intensity are observed and measured when the narrowed spectral emission band of the laser is tuned through a photo-absorption line of the photo-absorbing element. The purge chamber is purgeable with an inert gas such as argon or nitrogen gas for converting the calibration system into a clean output laser beam enclosure, and vice-versa.